A wide variety of systems for measuring liquid depth and/or content within a region such as a container or vehicular fuel tank have been developed over the years. Such systems commonly employ vertical displacement of a buoyant or float member disposed on the surface of the liquid in a water tank for example for operating a valve to maintain a constant surface level of liquid within the tank. Such float members have also commonly been linked to a variable resistor such as a potentiometer to provide an electrical output signal that varies with the height of the liquid surface within a tank or the like which, by a variety of means is converted to a visual indicator calibrated to display depth and/or content of the liquid within the region being monitored. Fluid measuring systems employing such float members and potentiometers have heretofore been susceptible to wear, sticking and breakage and thus have been found to present maintenance and accuracy problems.
More recently the effects of imparting a duty cycle or frequency change or phase shift or impedance change to an electrical circuit commonly incorporating an oscillator in response to movement of a core member within an electromagnetic field of a coiled current carrying conductor (generally called eddy current sensors) to variations in level or pressure of a liquid have been used to advantage in eliminating contact wear commonly effecting potentiometers used in such applications in the past.
An example of an eddy current sensor employing circuitry adapted to sense the position of a metallic object with respect to an electromagnetic field and provide either a digital or analog output indicative thereof is disclosed in U.S. Pat. No. 4,112,365, the disclosure of which is included herein by reference. An example of a liquid level monitor and controller employing an eddy current sensor having a core member moveable in response to changes in variations in liquid pressure to correspondingly alter phase of an electrical circuit is disclosed in U.S. Pat. No. 4,201,240, the disclosure of which is incorporated herein by reference. The core member, however, is of standard tubular design and is not provided with means for controlling its movement in response to pressure variations. An example of an eddy current type pressure transducer suitable for use in the system of the present invention utilizing movement of a metallic armature within an electromagnetic field of a coiled current carrying conductor in response to changes in liquid pressure to produce a frequency change in the current which is then converted by means of oscillator and buffer circuitry to provide an indication of fluid height is disclosed in my copending U.S. patent application Ser. No. 676,701 filed Nov. 30, 1984.
All of such eddy current type sensors however have heretofor been employed in systems adapted to monitor liquid surface level or pressure at a single location within a region and are thus unable to provide meaningful information as to averaged content or depth of a liquid within a region having an irregular bottom or sides or both resulting in different liquid depth across the region or, for example, in a fuel tank that is tipped back and forth resulting in almost instantaneous changes in depth of the fuel at all locations except along the rotational axis of the fuel within the tank. Such prior art eddy current type sensors have further featured conventional moveable metallic core members or armatures that have not been provided with means for controlling their movement in the manner desired to provide accurate measurement of content or flow of liquid contained in an irregular shaped region such as a fuel tank even where the measurement is taken at a single location within the region. Further, such prior art eddy current type sensors have not provided differential pressure measurement useful, for example, in providing flow rate of the fluid being monitored.